Water mimicry enables the solid solution of caffeine and theophylline sulphate and a new type of non-stoichiometric hydrate

A solid solution of caffeine and theophylline is realized as sulphate salt hydrate. The molecules enter the structure with one and two equivalents of water respectively, creating a novel type of non-stoichiometric hydrate. The solid solution is more thermally stable and enables an increased dissolution rate in water for caffeine and theophylline than the respective hydrated sulphate salts. In spite of increase solubility, the solid solution shows reduced permeability of caffeine through a synthetic skin membrane when tested against the physical mixture of the parent salts.